In currently used mobile radio telecommunication systems such as the existing GSM system, radio resources are limited due to the limited radio spectrum available and/or reserved for such purposes. The saving of radio resources is achieved by the usage of codecs (encoding-decoding devices) which operate at low bit rates. Namely, with such low-bit-rate codecs, the transmission rate on a radio interface is reduced to/expanded from 13 kbps in current GSM systems.
Furthermore, existing mobile radio telecommunication networks like GSM networks have to co-operate with existing public switched telephone networks PSTN such as the currently spreading ISDN network (Integrated Services Digital Network).
However, ISDN-PSTN networks primarily use a representation for speech data of 64 kbps in pulse code modulation (PCM), while the mobile radio telecommunication network uses the above mentioned 13 kbps representation.
Hence, interoperability at an interface I/F between the two types of networks has to be provided for, which is achieved by an interworking function unit also referred to as IWF unit or gateway unit (GW).
In case a call is forwarded from the ISDN-PSTN network to a mobile station MS as a radio terminal device of the radio telecommunication network (and vice versa), a speech coding functionality is required on both sides, i.e., on the network side and on the terminal device side. Namely, speech data of a call forwarded from the ISDN-PSTN network have to be coded in the network side for transmission via the radio interface (air interface Um), and if speech data of “an answer” are transmitted from the terminal device side, the speech data have likewise to be coded for transmission via the air interface.
Such a transmission of coded speech using time divisional multiplexing (TDM) between the radio access part or access network of the telecommunication network (e.g., a base transceiver station) and a speech codec part (e.g., transcoder and/or Transcoding Rate and Adaptation Unit (TRAU)) associated to, for example, a Mobile Services Switching Center (MSC) as a part of a core network of the telecommunication network, according to GSM, is effected using so-called TRAU frames (for details, reference is made to GSM 08.60).
Now, if a call is established between two terminal devices MS_A and MS_B, speech data transmitted there between are normally transcoded twice. Namely, firstly speech is encoded in the terminal device MS_A and subsequently decoded in the network. Thus, the speech data are present in the 64 kbps PCM format. Thereafter, the speech is encoded again in the network for transmission to the terminal MS_B, where it is decoded upon being received.
Thus, the coding is performed twice, while such double coding adversely affects the quality of transmitted speech, which of course, is undesirable.
FIG. 1 shows known telecommunication system according to GSM specification. A mobile services switching center MSC is connected to a transcoder TC via the A interface A-I/F. The transcoder TC in turn is connected to the base station controller BSC as a transceiver control device via an Ater interface Ater-I/F. The base station controller in turn is connected to a transceiver device or base station BS via an Abis interface Abis-I/F. Such a base station (base transceiver station) BS communicates with a subscriber terminal such as a mobile station MS via an air interface or radio interface RADIO-I/F also referred to as Um interface Um-I/F.
In the above described network, at least the MSC forms part of a so-called core network, while at least the BSC and BS constitute an access network (radio access network). As regards the transcoder TC, which provides the TRAU functionality that includes speech coding and data rate adaptation, according to GSM, this transcoder is logically associated to the access network side, i.e. to the base station system BSS (formed by the BSC and BS) communicating with the core network, i.e. the MSC via the A interface A-I/F as an open interface.
It is to be noted that the present description distinguishes between an open interface on one hand and a proprietary interface on the other hand. The term “proprietary interface” is intended to designate an interface specific for an individual network and thus dependent on a network operator, while the term “open interface” is intended to designate an interface independent of individual networks and thus independent of network operators that can be accessed from different networks, presumably also run by different operators.
Thus, referring back to the transcoder TC, the above means that the base station system BSS is controlling the operation of the transcoder TC. However, data such as speech and/or other data is present in a 64 kbit/s PCM mode at the A interface A-I/F. Transmission at such an elevated or high data rate, however, is rather expensive, and therefore, the transcoder is physically located at the core network side, i.e., close to the MSC as a core network control device, thereby reducing the data rate already in the core network. Thus, speech and/or other data to be transmitted are transmitted via a physically actually long transmission path from the transcoder and the base station controller BSC in coded format, which transmission path is referred to as Ater interface Ater-I/F.
Recently, however, new telecommunication systems are currently being developed, which systems are referred to as third generation systems (in short 3G systems). Specific examples for such 3G systems are the UMTS (Universal Mobile Telecommunication System) and/or the IMTS-2000 (International Mobile Telecommunication System for the year 2000).
In those 3G telecommunication networks, the transcoder TC is also logically associated to (and mostly also physically located in) the core network side and communicates with the corresponding 3G access network via an open Iu interface Iu-I/F. That is, data/speech is transmitted in coded format over this interface based on ATM transmission principles (Asynchronous Transmission Mode).
FIG. 2 shows a specific implementation of a telecommunication network, which is adapted to establish communication with a GSM based terminal such as a mobile station MS as well as with a third generation based terminal denoted in FIG. 2 by 3G_MS. Those parts which are similar or identical to those illustrated in connection with FIG. 1 explained above are denoted with the same reference signs and their explanation is not repeated in connection with FIG. 2.
The specific configuration shown in FIG. 2 illustrates a case, in which the 3G core network is based on GSM technology, i.e. uses a GSM based core network control device MSC. In this case, the same MSC can handle/manage communication via GSM as well as via 3G (e.g. UMTS) radio networks via the open A interface A-I/F.
Therefore, as illustrated in FIG. 2, there has to be provided an interworking unit IWU for the 3G part of the system, adapted to perform an adaptation between the A interface A-I/F and the Iu interface Iu-I/F. Thus, the 3G transcoder TC must be located in association with the interworking unit IWU, as shown in FIG. 2. (In an alternative case, (not shown in the figure), the transcoder could be located in association with the 3G radio network controller RNC without Iu interface).
Anyway, in any such a case the transcoder TC for the GSM part has to be purchased from the same manufacturer as the rest of the base station system BSS (comprising BSC & BS's), since the Ater interface Ater-I/F is a proprietary interface uniquely adapted to each respective manufacturer and/or network operator. However, for the transcoder for the 3G network part IWU/TC this is not the case, and this transcoder does not underlay special requirements in terms of the manufacturer which has to supply the transcoder.
Thus, due to the imposed restrictions as set out above, such a telecommunication system suffers from a reduced flexibility and a degraded compatibility of network components. In addition, different transcoding hardware resources have to be provided for.
In a current implementation, data are transmitted via the Ater interface for GSM in a data format used for the Abis interface, i.e. in TRAU frames. However, these frames have been specified only for TDM (PCM transmission) and the respective implementations are also proprietary.
Thus, also these implementations suffer from the above described drawbacks.